1. Field of the Invention
The present invention relates to a method of agglomerating finely ground polytetrafluoroethylene molding powder with or without a filler. More particularly, it relates to a novel method of agglomerating finely ground polytetrafluoroethylene molding powder having an average particle size of less than 100.mu. by subjecting it to high speed agitation in a medium comprising water and a hydrophobic organic liquid whereby excess agglomeration is controlled to obtain polytetrafluoroethylene agglomerated powder having narrow a particle size distribution for a short time.
2. Description of the Prior Art
It has been proposed to agglomerate finely ground powder in order to improve the processability of granular molding powder of polytetrafluoroethylene (hereinafter referred to as PTFE).
Agglomerated PTFE has excellent powder flowability and bulk density and when molded imparts excellent characteristics to the molded product. Moreover, agglomerated PTFE is advantageously used in automatic molding processes.
Various processes for agglomerating PTFE powder have been proposed, such as for example the agglomeration of PTFE by agitation in a slurry in water (U.S. Pat. No. 3,766,133 and U.S. Pat. No. 3,366,615), and the agglomeration of PTFE by wetting with an organic liquid (U.S. Pat. No. 3,265,679 and B.P. 1,100,388). Still another technique is agglomeration of PTFE by agitation of a slurry in a mixture of water and an organic liquid (U.S. Pat. No. 3,527,857 and B.P. 1,100,388).
Methods of agglomeration of a filled PTFE powder have been proposed by agitating the filled PTFE in water (U.S. Pat. No. 3,766,133) by wetting the filled PTFE with an organic liquid (B.P. 1,100,388).
The conventional methods of agglomerating PTFE powder have the following disadvantages. For example, in the method of agglomeration by wetting with an organic liquid excess agglomeration cannot be easily prevented thereby obtaining a lesser amount of the desired agglomerated PTFE and the method has been disadvantageous in the preparation of the agglomerated PTFE having narrow particle size distribution and relatively fine particles. When the agglomerated PTFE containing a filler is prepared, it has been required to give a post-treatment such as tumbling process for improving powder flowability so as to obtain suitable product.
In the method of agglomeration by agitation of a slurry in water, large excess power for agitation for long time has been required, and it has been difficult to obtain the agglomerated PTFE having relatively small particle size. It has been usually difficult to obtain the agglomerated PTFE containing a filler by the method of agglomeration by agitation of a slurry in water.
In the method of agglomeration by agitation of a slurry in a mixture of water and an organic liquid described in U.S. Pat. No. 3,527,857 and B.P. 1,100,388, the amount of the mixture of water and an organic liquid to PTFE powder has been too large whereby excess power for agitation has been required and control of particle size distribution has been difficult.